Tightened All the Bolts, but Still Loose?
Don’t worry—it’s probably not about brute force. The real issue might be the tightening sequence.
Let’s break down how to tighten bolts correctly in different situations to avoid loosening and ensure structural stability.
1. Why Do Bolts Loosen Even After Tightening?
When multiple bolts are involved, tightening one can affect the preload of the others nearby. This is known as elastic interaction.
For example, tightening the center bolt first can compress the joint further, causing a drop in preload on both sides. This interaction is especially noticeable in soft joints, such as those with gaskets or washers.
2. What’s the Correct Tightening Approach?
Ideally, specialized tools can be used to tighten all bolts simultaneously. However, in most real-world applications, due to limitations in tools, manpower, or space, bolts must be tightened one by one, which makes the tightening sequence critically important.
Scenario 1: With Locating Pins → Start Near the Pins
Locating pins help prevent misalignment and ensure assembly accuracy by fixing the relative position of components.
By tightening the bolts near the pins first, you can quickly establish the correct positioning baseline and prevent shifting during subsequent tightening steps.
Scenario 2: No Locating Pins? → Start with the Stiffest Section
Begin with areas that have high stiffness—such as thick metal, hard contact surfaces, or joints without gaskets—and then move on to softer sections, like thin walls, gasketed joints, or flexible materials.
This minimizes structural deformation and helps distribute the preload evenly across the joint surface for a stronger, longer-lasting connection.
Scenario 3: Complex Structures? → Let the Layout Decide
- Circular Bolt Patterns: Use cross patterns, star patterns, or quadrant tightening. These methods tighten bolts symmetrically, ensuring even preload distribution.
The cross-pattern is the most widely used and reliable approach.
- Rectangular Bolt Patterns: Use a spiral sequence, tightening from the center outward.
- Multi-Layer Bolt Groups: Divide into zones. Start with inner bolts, then tighten outward, allowing preload to radiate evenly.
- Standard Nodes: Tighten from the center of the node outward to the edges (see diagram for guidance).
- Box-Type Nodes: Follow the A → B → C → D sequence for systematic tightening.
- I-Beam Nodes: Follow a defined ① → ② → ③ → ④ → ⑤ → ⑥ sequence.
- I-Column Nodes: Similar to beam nodes—tighten the flanges first, then the web.
- Multiple Connected Nodes: Follow the rule: “Primary components first, secondary components later.” This applies to complex steel structures with multiple node connections.
3. Additional Key Tips to Avoid Loosening
Tighten in Stages
- Initial Tightening: 20%–30% of the target torque
- Intermediate Tightening: 50%–70% of the target torque
- Final Tightening: 100% of the target torque
Allow 1–2 minutes between each stage to let the stress distribute evenly.
Don’t Rush the Process
- For standard joints: 40–80 rpm
- For soft joints (rubber or plastic components): slower and steadier tightening
Also, perform a re-tightening within 10 minutes after the first round to further stabilize the connection.
Choose the Right Tools
- Pressure-sensitive film helps visualize force distribution, allowing for a better tightening strategy.
- Angle-tightening or bolt-stretch methods control the elongation of bolts more accurately than torque-based methods, leading to a more reliable clamping force.
If you’re wondering why bolts are loosening despite being fully tightened, take a closer look at your tightening sequence, staging, speed, and tool selection.
A well-planned bolt tightening strategy is key to improving assembly quality, ensuring joint durability, and minimizing long-term maintenance.
Built to Perform. Engineered by Professionals.
At Suzhou Bilateral Import & Export Co., Ltd, we are committed to delivering fastening solutions that meet the highest standards of strength, precision, and durability. Whether you’re installing solar structures, structural steel joints, or precision equipment, our stainless steel fasteners are trusted by engineers worldwide for secure, long-lasting performance.
Need expert advice on fastener installation or custom bolt solutions?
Contact our technical team or explore our product range at www.szbolts.com.
Contact Mail: sales@szbilateral.com
